Disposable absorbent articles such as diapers and adult incontinence products are well known in the art. Such disposable articles collect and retain urine and fecal material deposited thereon by the wearer.
Nonwoven fabrics made of synthetic fibers are commonly applied in absorbent articles, for example, as topsheet material or as core wrap to enclose the storage layer of the absorbent core. Such nonwoven fabrics are usually hydrophobic. However, for many applications in hygiene products it is necessary to have hydrophilic nonwoven. Therefore the nonwoven fabric has to be treated accordingly.
A common method for rendering nonwoven fabrics hydrophilic is coating the surface of the nonwoven with hydrophilic surfactants. As this coating does not lead to a tight, chemical bond between the nonwoven and the surfactant, the surfactant can be washed off during use when the absorbent article is wetted. The decrease in liquid strike through time is a desirable effect when the nonwoven is coated with surfactant. Liquid strike through refers to liquid passing through the nonwoven fabric with liquid strike through time referring to the time it takes for a certain amount of liquid to pass through the nonwoven. However, as the surfactant is washed off when coated nonwoven fabrics are exposed to the liquid, the strike through time in the next gushes is increased again. This results in performance reduction during use for diapers comprising those nonwoven fabrics. Furthermore, at the same time as liquid strike through time decreases due to use of surfactants, surface tension of the liquid, which was in contact with the nonwoven fabric, is reduced. This reduction is undesirable, because it can cause increased urine leakage in a diaper. On the other side, any surfactants leading to reduced strike through time also reduces surface tension of the wash off.
Examples of typical surfactants are described in WO 93/04113 entitled “Method for hydrophilizing absorbent foam materials” and in WO 95/25495 entitled “Fluid acquisition and distribution member for absorbent core”.
Another possibility to render a nonwoven fabric hydrophilic is by applying corona and plasma treatment.
Plasma is an ionized form of gas that can be obtained by ionizing a gas or liquid medium. Plasmas are widely used for the treatment of organic and inorganic materials to promote adhesion between various materials. Polymers that have chemically inert surfaces with low surface energies do not allow good coatings with bondings and adhesives. Thus, these surfaces are treated to make them receptive to bonding with other substrates, coatings, adhesives and printing inks. A method for producing plasma is described in U.S. Pat. No. 6,118,218 entitled “Steady-state glow-discharge plasma at atmospheric pressure”.
Corona discharge is an electrical phenomenon, which occurs when air is exposed to a voltage potential high enough to cause ionization, thereby changing it from an electrical insulator to a conductor of electricity.
However, corona and plasma treatment lead to low coating durability upon storage of material, i.e., hydrophilicity decreases over time.
WO 00/16913 entitled “Durably wettable, liquid pervious webs” and WO 00/16914 entitled “Durably wettable, liquid pervious webs prepared using a remote plasma polymerization process” disclose webs with a hydrophilic coating applied by a plasma polymerization process. However, the drawback of this process is, that commercial application is constrained, because it is very slow and cannot be carried out continuously but needs a batch process.
Thus, there is a need for a hydrophilic coating of a nonwoven, which is durable upon storage, is not easily washed off when wetted and allows to achieve fast liquid strike through in multiple exposures to liquid without surface tension reduction of wash-off.
Methods of chemically grafting hydrophilic monomers are known in the art. For example U.S. Pat. No. 5,830,604 entitled “Polymeric sheet and electrochemical device using the same” issued to Raymond et al.; U.S. Pat. No. 5,922,417 entitled “Polymeric sheet” issued to Raymond et al.; and WO 98/58108 entitled “Non-woven fabric treatment” all refer to a process to produce nonwovens for use as separator in electrochemical devices such as batteries.
EP 1 164 157 A1 entitled “Method of modifying polymeric material and use thereof” discloses a method of modifying polymeric material, which comprises the steps of an activation treatment and a hydrophilic polymer treatment in this order. The method optionally further comprises a solvent treatment, which is carried out prior to the activation treatment and/or further comprises monomer grafting carried out after the hydrophilic polymer treatment. The disadvantage of this method is, that it is very complex and comprises numerous steps. Moreover, treating a nonwoven with water-soluble polymers lead to a reduction in surface tension of water when such nonwoven is exposed to water.
It is one objective of the present invention, to provide absorbent articles, which comprise nonwoven fabrics with a durable hydrophilic coating.
It is a further objective of the present invention, to provide absorbent articles, comprising nonwoven fabrics with hydrophilic coatings, which are not washed off.
Moreover, it is an objective of the present invention to provide absorbent articles, which comprise nonwoven fabrics with low strike through times even after several gushes and which at the same time do not have a reduced surface tension when contacted with aqueous liquids.
It is a still further objective of the present invention to provide a process for making a nonwoven fabric, which are suitable for absorbent articles having the desired properties.